Previous approaches are known for establishing compression losses at individual cylinders of an internal combustion engine by evaluating a crankshaft sensor signal. In doing this, certain segment travel times are measured and evaluated in the control unit by measuring teeth of a crankshaft sensor wheel, making it possible to infer the time interval that a crankshaft requires to pass a predefined angle segment. If compression losses occur in the monitored cylinder, it takes the crankshaft less time to pass an angle segment before the ignition top dead center of the piston in the cylinder than would be the case when the closure or sealing of the cylinder is error-free (i.e., “okay”). This dead center is also referred to below as “ITDC” to adequately distinguish it from the top dead center between power strokes in which the piston presses combustion gases out of the cylinder and subsequently draws a fresh fuel mixture into the cylinder. The “ignition top dead center,” on the other hand, refers to the dead center at which the piston switches from the compression power stroke to the power stroke for igniting the fuel mixture and therefore to the power stroke for initiating the expansion phase. Note that in the description below, this specific ignition top dead center is meant when referring only to a “top dead center.”
This shorter time required by the piston to pass the aforementioned angle segment when a leak occurs results from the fact that the compression work to be performed by the piston is reduced, and the piston (and therefore also the crankshaft) does not needed to be braked as forcefully. The opposite effect is obtained accordingly in the expansion phase after the ITDC. Here, the segment travel time increases, since the compression is no longer as substantial as in the case of error-free cylinder closure. These times are currently evaluated separately and directly before the ITDC and after the ITDC, and errors are detected on the basis of a relative monitoring of the individual cylinders. However, a procedure of this type is problematic if tolerances occur in the measuring teeth of a sensor wheel which is attached to the crankshaft and whose measuring teeth are used to determine the time intervals which the crankshaft requires to pass the corresponding angle segments before and after the ITDC. Under some circumstances, the tooth times to be evaluated are corrupted by these sensor wheel tolerances to such a great extent that errors may be incorrectly detected on the cylinders or, alternatively, at high tolerances, a very high error detection threshold must be selected, which causes the diagnostic quality to suffer in the event of an error. As a result, the diagnostic quality is currently dependent on the sensor wheel tolerances.
An object of the present invention is therefore to provide a way to detect leaks in a cylinder of the internal combustion engine better and more economically, in particular also to compensate for sensor wheel tolerances using a simple and cost-effective approach.